Plasmonic Nanohole Arrays on Top of Porous Silicon Sensors: A Win–Win Situation

Label-free optical sensors are attractive candidates, for example, for detecting toxic substances and monitoring biomolecular interactions. Their performance can be pushed by the design of the sensor through clever material choices and integration of components. In this work, two porous materials, namely, porous silicon and plasmonic nanohole arrays, are combined in order to obtain increased sensitivity and dual-mode sensing capabilities. For this purpose, porous silicon monolayers are prepared by electrochemical etching and plasmonic nanohole arrays are obtained using a bottom-up strategy. Hybrid sensors of these two materials are realized by transferring the plasmonic nanohole array on top of the porous silicon. Reflectance spectra of the hybrid sensors are characterized by a fringe pattern resulting from the Fabry–Pérot interference at the porous silicon borders, which is overlaid with a broad dip based on surface plasmon resonance in the plasmonic nanohole array. In addition, the hybrid sensor shows a significant higher reflectance in comparison to the porous silicon monolayer. The sensitivities of the hybrid sensor to refractive index changes are separately determined for both components. A significant increase in sensitivity from 213 ± 12 to 386 ± 5 nm/RIU is determined for the transfer of the plasmonic nanohole array sensors from solid glass substrates to porous silicon monolayers. In contrast, the spectral position of the interference pattern of porous silicon monolayers in different media is not affected by the presence of the plasmonic nanohole array. However, the changes in fringe pattern reflectance of the hybrid sensor are increased 3.7-fold after being covered with plasmonic nanohole arrays and could be used for high-sensitivity sensing. Finally, the capability of the hybrid sensor for simultaneous and independent dual-mode sensing is demonstrated.

To deposit a loosely packed hexagonally ordered colloidal array on top of glass cover slips, the purified PS@polyNIPAM core-shell particle dispersion was mixed with ethanol (99.8 %) in a volumetric ratio of 1:1. 10 µL of the resulting mixture was spread over a hydrophilic glass coverslip (24 x 24 mm, cleaned with piranha solution (3:1 (v:v) mixture of concentrated H 2 SO 4 : H 2 O 2 (30%)) for at least 1 h) which was afterwards slowly dipped into a MilliQ water bath (500 mL water in a crystallizing dish with an internal diameter of 11 cm) at RT. Thereby, PS@polyNIPAM particles were transferred to the air/water interface. After addition of 4 µl of sodium dodecyl sulfate (10 weight% in MilliQ water) a well-ordered hexagonal array of PS@polyNIPAM particles is formed which can be lifted-off the interface using highly hydrophilic glass coverslips (pre-cleaned in piranha solution: conc. H 2 SO 4 : H 2 O 2 (30%) 3:1 (v:v)). Best results were achieved by slowly immersing the glass cover slip at an angle of ~60 ° to the water surface and slowly removing it from the water at the same angle. Afterwards, the samples were dried in air at RT. The extraordinary properties of the S-3 PS@polyNIPAM particles led to a loosely packed, but well-ordered hexagonal colloidal array which was directly utilized as mask for the deposition of a gold film.
For providing adhesion between glass surface and gold film, the glass coverslips decorated with PS@polyNIPAM core-shell particles were first functionalized with APTES. Briefly, 30µL of APTES were mixed with 2 ml of isopropanol. The samples together with a small container filled with 150 µL of diluted APTES were placed in a 150 mL glass jar. The jar was closed and placed inside a preheated oven at 110°C for 1 h. The samples were taken out and washed thoroughly with MilliQ water. After drying under a stream of N 2 the samples were baked once more at 110°C for 1 h. Then, a thin layer of gold (~5 nm thickness) was deposited on the samples using a Quorum technologies Ltd. (Q150R ES) sputter coater.
To remove the colloidal mask the samples were first submerged in toluene at 8°C for at least 2 h and subsequently ultrasonicated in toluene at ~ 13°C for 60 min. After drying the samples in air the same procedure was carried out using a methanol : MilliQ water mixture (95:1, v:v) as immersion medium instead of toluene. The samples were washed with ethanol and dried in a stream of N 2 leaving behind a thin gold layer patterned with holes in a hexagonal array. The thickness of the gold layer was increased to the desired height using electroless gold deposition. In an open square plastic box (2.6 x 2.6 x 0.8 cm) 2 ml of an aqueous solution of HAuCl 4 * 3H 2 O (5 mg/mL) and 33 µl of an aqueous solution of hydroxylamine hydrochloride (5.54 mg/ml) were mixed. The samples were placed face down onto the solution. The reaction was carried out for 30 min on an orbital shaker (slow motion). The resulting plasmonic nanohole arrays were removed from the gold salt solution, rinsed with MilliQ water and dried in air.

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Assembly of hybrid sensor composed of porous silicon and plasmonic nanohole arrays Figure S1. Schematic representation of the fabrication of the hybrid sensor. The plasmonic nanohole array was placed in 0.1 M aqueous NaOH solution overnight to break the covalent bonds between glass substrate and gold film provided by APTES. a) The nanohole array on the glass substrate is immersed vertically in the MilliQ water bath. b) The nanohole array tends to float on the water surface due to its light weight, while the glass substrate sinks to the bottom of the water bath. c) The porous silicon substrate is immersed in water and placed directly under the gold layer. d) Finally, the porous silicon substrate is lifted out of the water together with the nanohole array. The sample is dried vertically in air and tempered at 45°C for 24 hours. Similar approaches have already been described several times in the literature. [3][4][5] S-5 Supporting figures: Figure S2. Radial distribution function of a plasmonic nanohole array. The center-to-center distances of the holes are well-defined and an averaged value can be obtained from the first sharp maximum (413 nm ± 2 nm). The diameter of hexagonally ordered domains can be estimated at 10 lattice constants.

Transmittance/ reflectance comparison
The plasmonic nanohole arrays are fabricated on a glass substrate. To obtain a reference sample consisting of a plasmonic nanohole array on a Si substrate, we used the same procedure that was used to transfer the plasmonic nanohole array onto porous silicon substrates. More specifically, we used a piece of polished silicon wafer (the same type used to prepare the porous silicon samples) oxidized at 600°C for 1 hour and functionalized with APTES (as described in the section on PSi preparation). Figure S5 a) shows the reflectance spectrum of the plasmonic nanohole array recorded in air after transfer to the silicon wafer. Obviously, the surface plasmon resonance (SPR) of the plasmonic nanohole array cannot be detected with the optical setup used before. It is now important to note that the SPR property has not disappeared, but has shifted to the UV region. This is probed by changing the refractive index of the surrounding medium from air to an aqueous solution of 50% of sucrose (w/w) as is shown in figure S5 b). Due to the increase of the refractive index of the surrounding medium the LSPR shifts to the visible range (~593 nm) as is highlighted by the red arrow. Therefore, transmittance spectra of the plasmonic nanohole arrays on glass substrates were recorded in order to determine the sensitivity of this structure.  S-9 Determination of sensitivities -raw data Figure S6 shows the raw data of the EOT shifts of 3 porous silicon sensors and 3 hybrid sensors. These shifts were calculated as the difference of the EOT of the sensors immersed in different aqueous sucrose solutions (EOT sn ) minus the EOT of the sensors immersed in water (EOT w ). The intensity of the EOT peak was also used as a transduction signal. The raw signals in sensors with and without plasmonic nanohole array on porous silicon monolayers are shown in Figure S7. The change in intensities was calculated as the intensity of the EOT peak of the sensors immersed in sucrose solutions (IntEOT sn ) minus the intensity of the EOT peak of the sensors immersed in water (IntEOT w ).   Figure S8. Reproducibility of the optical response of one sensor (porous silicon (orange) and hybrid sensor (green)) for different sucrose solutions. Error bars represent the standard deviation for changes in the optical signal corresponding to one sucrose solution (shown in black, almost invisible due to their small values).

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The SPR position shifts of hybrid sensors and plasmonic nanohole arrays on glass are compared and shown in Figure S9. The spectral shifts were calculated as the wavelength of the SPR of the sensors immersed in aqueous sucrose solution (λsn) minus the wavelength of the sensor exposed in water (λw). Figure S9. Changes in the spectral position of the SPR of plasmonic nanohole arrays on glass substrates (AuArr_01-AuArr_03) and plasmonic nanohole arrays on porous silicon monolayers (PSI_AuArr_01-PSI_AuArr_03).

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Dual mode sensing experiment Figure S10. Dual-mode sensing with a hybrid sensor consisting of a plasmonic nanohole array on porous silicon monolayers: Repetition with a different hybrid sensor.